LED (light-emitting diode) lighting devices provided with a step-down chopper are known, one of which is disclosed in, for example, patent document (Japanese Patent Publication No. 4123886). In this type of LED lighting device, a resistor element having a low resistance is connected between a FET serving as a first switching element and a first inductor, and this resistor element is connected between the base and the emitter of a bipolar transistor serving as a second switching element. The collector of the transistor is connected to the gate terminal of the FET.
When the FET is turned on, a current flows from a direct-current power supply via the resistor element, the first inductor and a capacitor connected parallel to an LED circuit serving as a load. When this current gradually increases and a voltage across the resistor element reaches a bias that allows the transistor to operate, the transistor is turned on, and thus the FET is turned off. Since the voltage across the resistor element is the base bias of the transistor, and this voltage reaches a predetermined voltage to allow the turning on of the transistor and thus the turning off of the FET, it is possible to accurately have a timing of the turning off without the timing being affected by a voltage induced by the second inductor. That is, it is possible to accurately perform the switching operation of the FET at all times. Then, when the charging voltage of the capacitor is equal to or more than the forward voltage of the LED circuit, a current flows through the LED circuit, with the result that the LED included in the LED circuit starts to light.
Since, in the case of a silicon transistor, a base bias for allowing the transistor to be turned on is so low as to be 0.5 volts, almost no electric power is consumed by a resistor element, and thus it is possible to prevent unnecessary power consumption as much as possible.
However, in the conventional LED lighting device, it is required to further reduce the power loss of the resistor element connected in series with the first switching element. Moreover, since the temperature characteristic of the first switching element is determined by the temperature characteristic of the transistor, it is disadvantageously difficult to provide a desired temperature characteristic for the first switching element.
It is an object of the present invention to provide an LED lighting device that can further reduce the power loss of impedance means connected in series with a switching element serving as a step-down chopper and that has a satisfactory temperature characteristic and a small amount of variation in output current.